1. Field of the Invention
The present invention concerns temperature-compensated piezoelectric oscillators, namely piezoelectric oscillators comprising a system for regulating frequency according to temperature.
2. Description of the Prior Art
Like standard piezoelectric oscillators, temperature-compensated oscillators consist of an amplifier which has its output connected to the input through a circuit comprising an oscillating piezoelectric resonator, the gain of the system being in a ratio of more than 1 in order to meet the conditions of self-oscillation. In this case, the frequency compensation is obtained by connecting, in series with the piezoelectric resonator, an element with a capacitance that is variable according to the voltage applied to its terminals, for example an element such as a variable capacitance diode, and by creating the voltage to be applied to the variable capacitance diode (or compensation voltage) in a compensation circuit including an element sensitive to temperature.
Temperature compensation systems make it possible to obtain a frequency that is practically stable whatever the temperature at the output of the oscillator. However, piezoelectric oscillators are subject to an ageing phenomenon, i.e. their oscillation frequency changes with time. It is therefore necessary to reset the oscillator as a function of time. This resetting consists in modifying the impedance in series with the piezoelectric resonator so as to obtain the nominal frequency of the oscillator. When variable capacitance diodes are used, the resetting operation consists in modifying the voltage at its terminals. Now, this resetting modifies the temperature compensation.
For, as shown schematically in FIG. 1, one embodiment of a resetting circuit used in a temperature-compensated piezoelectric oscillator is formed by a potentiometer Pe, mounted in series between the supply voltage V.sub.A and one terminal of a resistor R, the other terminals of which is grounded. The middle terminal of the potentiometer Pe is connected by means of a resistor R' to the cathode of the variable capacitance diode D. Furthermore, in the embodiment shown, the anode of the variable capacitance diode D is connected to the circuit CT for the regulation of frequency according to temperature. In a known way, the variable capacitance diode is mounted in series with the piezoelectric resonator P and the amplifier A, the output of which is looped to the anode of the variable capacitance diode D. Thus, when the potentiometer is made to vary in order to achieve the frequency resetting of the oscillator, the bias voltage of the variable capacitance diode is modified. Now, as shown in FIGS. 2A and 2B, the capacitance/voltage characteristic does not provide for obtaining a linear frequency variation as a function of its bias voltage. The result of this, therefore, is a modification of the slope P0 of the transfer function, which becomes the slope P1. This modification of the slope implies a rotation of the compensation curve as shown in FIG. 3 which concerns a frequency resetting of .+-.5.10.sup.-6 in the rated frequency F.sub.O. Furthermore, the coefficient of temperature of the variable capacitance diode varies according to the bias voltage (FIG. 2C), thus accentuating the rotation effect of the compensation curve.
In this FIG. 3, the straight line a at F.sub.O represents the ideal compensation at the time T.sub.O, the curves b and b' represents the frequency coming from the oscillator after positive or negative resetting for a frequency drift of .+-.5.10.sup.-6 FO. This distortion of the compensation curve have many drawbacks. In particular, the oscillator may be taken beyond the characteristics laid down by the specification.